1. Field of the Invention
The present invention relates generally to an arc welding method suited for factory automation, and more particularly to an arc welding method suited for use in welding a material such as, for example, aluminum or magnesium, which requires the removal of an oxide layer during welding. The present invention also relates to an arc welding machine for effecting this method.
2. Description of the Prior Art
FIG. 1 schematically depicts a conventional arc welding machine for effecting arc welding processes in an inert gas (not shown) with an alternating current being supplied from a power supply system 4 so as to flow between an electrode 1 and an object 3 to be welded. In a period represented by T.sub.ED, the electrode 1 is negative whereas the object 3 is positive. This period is characterized in that the penetration is deep, the rate of consumption of the electrode 1 is small, and arc sounds are small, as shown in Table 1.
On the other hand, in a period represented by T.sub.EF, the electrode 1 is positive whereas the object 3 is negative. When the material of a base is aluminum, magnesium or the like, an oxide obstructs welding. The reason for this is that the melting point of the oxide is higher than that of the base. The period T.sub.EP is characterized by an oxide removal action.
TABLE 1 ______________________________________ Oxide Layer Consumption of Arc Removal Action Penetration Electrode Sound ______________________________________ T.sub.EN No Deep Small Small ##STR1## ##STR2## T.sub.EP Yes Shallow Large Large ##STR3## ##STR4## ______________________________________
As described above, in applications where the melting point of an oxide is higher than that of a base, welding is performed by the use of an alternating current with T.sub.EN and T.sub.EP being alternated. In general, the alternating current required for the welding has a frequency of about 100 Hz, and the magnitude thereof is slightly greater during T.sub.EN than during T.sub.EP.
However, this kind of conventional arc welding method employing an alternating current during welding is not suited for use in fillet welding or Uranami welding (penetration welding) because the directivity of arcs is not stable. Furthermore, higher harmonics caused by the inversion enlarges arc sounds, and the electrode heating action during T.sub.EN results in rapid consumption of the electrode.